Semiconductor memory devices are rapidly-accessible memory devices. In a semiconductor memory device, the time required for storing and retrieving information generally is independent of the physical location of the information within the memory device. Semiconductor memory devices typically store information in a large array of cells. A group of cells are electrically connected together by a bit line, or data line. An electrical signal is used to program a cell or cells.
Computer, communication and industrial applications are driving the demand for memory devices in a variety of electronic systems. One important form of semiconductor memory device includes a non-volatile memory made up of floating-gate memory cells called flash memory. Computer applications use flash memory to store BIOS firmware. Peripheral devices such as printers store fonts and forms on flash memory. Digital cellular and wireless applications consume large quantities of flash memory and are continually pushing for lower voltages and power demands. Portable applications such as digital cameras, audio recorders, personal digital assistants (PDAs) and test equipment also use flash memory as a medium to store data.
Data values stored in memory cells are sensed in order to provide the data to an external device, such as a processor or memory controller. In a sensing operation, a read voltage is applied to a control gate of a memory cell and the cell is coupled to a sensing device. In a first state, the memory cell will activate or become conductive. This is often referred to as an erased state and often corresponds to a data value of 1. In a second state, the memory cell will not activate in response to the read voltage and will remain essentially nonconductive. This is often referred to as a programmed state and often corresponds to a data value of 0. The differences in the states of the memory cell are dependent upon the nature of the cell, but generally result from differences in threshold voltage (Vt) of a field-effect transistor. One example includes the addition or removal of charge from a floating gate of a floating-gate memory cell or a trapping layer of an NROM cell. Another example includes the alteration of the remanent polarization of a ferroelectric layer in a ferroelectric memory cell. Other nonvolatile memory cells are also known in the art whose data value is indicated by the threshold voltage of a transistor.
In flash memory, and NAND flash memory in particular, sensed data is latched for page read-out. This latched data should store the data value while data is transferring to an output pin of the device. When switching an output buffer, however, ground noise and power noise is often generated. This is especially true when the output data is changing from one logic level to the other, e.g., high to low or low to high. Noise propagating to the internal circuits of the memory device can create malfunctions, leading to data errors or worse.
NAND sensing devices typically include two latches for holding sensed data, i.e., a data latch and a cache latch, because sensing times are typically much longer than those seen with NOR memory. For example, NAND sensing times may be on the order of 10–20 μs while NOR sensing times may be on the order of 55–90 ns. Sensed data is latched in the data latch and then passed to the cache latch. While reading data from the cache latch, the data latch can be used to latch a data value from the next addressed memory page. In this manner, the relatively slow sensing times can be hidden from a user when reading several consecutive pages.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternative sensing devices for integrated-circuit memory devices, memory devices containing such sensing devices, and methods of their operation.